Clinical application of laser evoked potentials using the Nd:YAG laser

doi:10.1016/S0987-7053(02)00297-6

Neurophysiologie Clinique/Clinical Neurophysiology

Volume 32, Issue 2, April 2002, Pages 91-98

https://doi.org/10.1016/S0987-7053(02)00297-6 Get rights and content

Abstract

The clinical interest of a new type of laser evoked potentials (LEPs) using Nd:YAG laser was assessed in the diagnosis of peripheral neuropathies affecting the small-diameter nerve fibres, and of spinal cord lesions, affecting the spinothalamic tract. Twelve patients aged from 26 to 79 years with sensory neuropathies (n = 6) or spinal cord lesions (n = 6) underwent neurophysiological examination of the lower limbs comprising quantitative sensory testing, i.e., the determination of vibratory and thermal thresholds (VT and TT), somatosensory evoked potentials (SEPs) to electrical stimulation and Nd:YAG LEPs. VT and SEPs were used to assess large-diameter afferent nerve fibres and the lemniscal pathways while TT and LEPs were used to assess small-diameter afferent nerve fibres and the spinothalamic tract. In addition, patients with peripheral neuropathy underwent also standard nerve conduction studies to explore large fibres and the recording of sympathetic skin responses (SSRs) to explore small fibres, whereas motor evoked potentials were performed in patients with spinal cord lesion. LEPs were absent bilaterally in all patients with polyneuropathy, even when TT remained within the normal limits and SSRs were present. LEPs were absent after stimulation of the affected limb in all patients with a spinal cord lesion, and allowed to detect subclinical contralateral lesion in two cases. LEPs following Nd:YAG laser stimulation are sensitive in the diagnosis of peripheral and/or central nervous system disorders and they give complementary information as compared to routine electrophysiological tests.

Résumé

L’intérêt clinique d’un nouveau type de potentiels évoqués laser (PEL) obtenus par stimulation utilisant un laser Nd:YAG a été évalué chez des patients présentant une neuropathie périphérique touchant les fibres nerveuses de petit calibre ou une atteinte médullaire touchant la voie spino-thalamique. Douze patients âgés de 26 à 79 ans présentant soit une neuropathie (n = 6) soit une atteinte médullaire (n = 6) ont fait l’objet d’une étude neurophysiologique des membres inférieurs incluant la quantification des seuils de sensibilité, vibratoire et thermique (SV et ST), l’enregistrement des potentiels évoqués somesthésiques à la stimulation électrique (PES) et les PEL. Les SV et les PES exploraient les grosses fibres nerveuses et les voies lemniscales, tandis que les ST et les PEL exploraient les petites fibres nerveuses et les voies spino-thalamiques. Par ailleurs, une étude classique des conductions nerveuses et l’enregistrement des réponses cutanées sympathiques (RCS) ont été effectués chez les patients présentant une neuropathie périphérique pour apprécier respectivement les fibres de gros diamètre et de petit diamètre, alors que des potentiels évoqués moteurs étaient réalisés chez les patients ayant une atteinte médullaire. Les PEL étaient absents bilatéralement chez tous les patients présentant une neuropathie périphérique, même lorsque les ST et les RCS étaient normaux. Les PEL étaient absents après stimulation du membre atteint chez tous les patients présentant une atteinte médullaire, tandis qu’une atteinte infra-clinique controlatérale était mise en évidence chez deux patients. Ces résultats témoignent de la sensibilité des PEL par stimulation Nd:YAG pour diagnostiquer une atteinte périphérique et/ou centrale du système nerveux. La réalisation des PEL apporte des informations complémentaires essentielles aux tests électrophysiologiques usuels.

Introduction

The laser stimulation and the cortical responses evoked by it, so-called ‘laser evoked potentials’ (LEPs), were introduced in research in the mid ‘70s in the attempt of better understanding the mechanisms of nociception [14]. The most important achievement from the application of LEPs as a clinical tool is the possibility to investigate objectively some peripheral and central pathways which were prior unexplored in clinical neurophysiology practice 4, 9, 18, 19. Compared to electrical stimulation used for standard somatosensory evoked potential (SEP) testing, which stimulates the large diameter, A-beta nerve fibres, the interest of laser stimulation is to stimulate the small-diameter, thinly myelinated, A-delta nerve fibres and the unmyelinated C-fibres 12, 26, 34, 41, 42, 51. Regarding the central nervous system pathways, the LEPs allow the investigation of the spinothalamic tract 16, 29, 46, whereas the dorsal columns are explored by electrical SEPs. At present, the clinical application of LEPs for the diagnosis of peripheral or central nervous system diseases is just emerging 9, 13, 31, 34.

A CO₂ laser (wavelength 10.6 μm), with pulse duration ranging between 10 ms and 60 ms, is classically used to perform LEPs 10, 11, 30. Lasers with longer pulse duration (200 ms) have been proposed as stimulant, i.e. argon laser (wavelength 0.5 μm) 3, 5, 7 and diode laser (wavelength 0.98 μm) [24]. In contrast, a Thulium (Tm):YAG laser (wavelength 2 μm) with short pulse duration of 1–2 ms was recently introduced to produce LEPs 17, 35, 49. Brief CO₂ laser pulses (100 ns) [44] and Neodinium (Nd):YAG laser pulses [50] have been used for reaction times and pain threshold measurements, and we were the first to apply a laser beam with an ultra-short pulse duration (5 ns) delivered by a Q-switched Nd:YAG laser (wavelength 1.06 μm) to obtain LEPs. This original approach has been validated in a previous study performed on healthy subjects [38].

A brief laser pulse may be an advantage by leading to shorter activation times, better synchronization of the afferent volley and greater spatial-temporal summation at central synapses. This may contribute to the occurrence of LEPs with shorter latency following a ns-range Nd:YAG laser pulse than following a ms-range CO₂ laser pulse. However, a high peak power emission should be available to reach the amount of energy necessary to activate the small nerve fibres and/or their receptors. This condition is fulfilled by the Q-switched arrangement of the Nd:YAG laser. The high power emission obtained with a Q-switched or a mode-locked Nd:YAG laser is able to induce a strong electric field in tissue, that could be the main mechanism to activate the mechano-thermal nociceptors and to produce a pinprick pain sensation, preferentially to the radiant heating which is rather provoked by a CO₂ laser stimulation. Other remarks are inherent in the use of a Nd:YAG laser. For instance, the Nd:YAG laser beam penetrates deeper in the skin layers than the CO₂ or even the Tm:YAG beam, since skin transmittance increases as laser wavelength shortens within the near-infrared range. Therefore the energy density at the air/skin interface necessary to produce LEPs may be slightly higher for the Nd:YAG laser than for the CO₂ laser (10–14 mJ mm^–2 vs 8–12 mJ mm^–2) 6, 10, 21, 45, but owing to a lower tissue absorption, the energy dissipation per unit of volume into the skin may be weaker for the Nd:YAG laser than for the CO₂ laser.

Besides the fact that the exact mechanisms of LEP production by Nd:YAG laser remain to be precised, the aim of the present study was the attempt to verify the clinical usefulness of the Nd:YAG LEPs in the investigation of peripheral neuropathies and spinal cord lesions.

Section snippets

Patients and methods

Six patients with a suspected peripheral sensory neuropathy involving small diameter nerve fibres, but no clinical signs or history of central nervous system disorder (3 women and 3 men, mean age: 64 years, range: 48–79 years) and 6 patients with a suspected unilateral spinothalamic tract lesion (1 woman and 5 men; mean age: 42 years, range: 26–63 years) but no clinical signs or history of a peripheral nervous system disorder were included.

The diagnosis of peripheral neuropathy or spinothalamic

Results

The electrophysiological findings are presented in Table 2 and examples of LEPs in Fig. 1. LEPs following Nd:YAG stimulation of the feet were bilaterally absent in all patients with peripheral neuropathy and at least unilaterally in all patients with spinothalamic tract lesion. LEPs were found bilaterally absent in one patient (n° 9) with spinal lesion. Another one (n° 8) presented absent LEPs for one side, whereas LEPs of the other side had low amplitude (4 μV) and prolonged latency (255 ms)

Discussion

The use of Nd:YAG laser stimulation to produce LEPs appears to be feasible in clinical practice, and extremely sensitive both to the detection of peripheral small-diameter nerve fibre dysfunction and to that of central spinothalamic tract lesion. This observation illustrates the interest of the use of LEPs in the diagnosis of neurological disease.

Since LEPs evaluate the small-diameter nerve fibres and the spinothalamic tracts, then similar results of LEPs and thermal sensory testing could be

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Le diagnostic des neuropathies des petites fibres (NPF) est un défi pour la neurophysiologie clinique. Les techniques classiques d’étude des conductions nerveuses sont inappropriées et divers tests neurophysiologiques ont été proposés à cet effet. Dans cette étude, nous avons comparé la valeur diagnostique de cinq de ces tests chez 87 patients avec une NPF cliniquement définie (n = 33) ou possible (n = 54), liée à une neuropathie amyloïde secondaire à une mutation du gène de la transthyrétine ou à une gammapathie monoclonale (n = 30), à un syndrome de Sjögren primaire (n = 20), la maladie de Fabry (n = 2), ou une cause indéterminée (n = 35). Les tests neurophysiologiques, effectués aux quatre extrémités (mains et pieds), comprenaient une étude quantifiée de la sensibilité thermique, avec détermination des seuils de détection au chaud et au froid (SDC, SDF), l’enregistrement des potentiels évoqués par stimulation laser (PEL) et des réponses cutanées sympathiques (RCS), et la mesure de la conductance électrochimique cutanée (CEC) utilisant un appareil Sudoscan^®. Tous les patients avec NPF cliniquement définie et 70 % des patients atteints d’une possible NPF avaient au moins un test anormal. Les PEL étaient le test le plus sensible (modifiés chez 79 % des patients avec au moins un test anormal), suivis par la CEC (61 %), le SDC (55 %), la RCS (41 %), et enfin le SDF (29 %). La combinaison des PEL, évaluant les fibres A-delta sensitives, du SDC, évaluant les fibres C sensitives, et de la CEC, évaluant les fibres C autonomes, semble représenter une approche pertinente pour le diagnostic des NPF. Comparés à la RCS et au SDF, ces trois tests, PEL, CEC et SDC, avaient une sensibilité diagnostique significativement meilleure dans cette étude et leur combinaison améliorait davantage la précision du diagnostic de NPF.
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2014, Pratique Neurologique - FMC
Les potentiels évoqués laser (PEL) constituent une technique électrophysiologique fiable, non invasive permettant d’étudier spécifiquement et de façon objective les nocicepteurs. Les PEL sont indiqués lorsqu’il y a forte suspicion de neuropathie à petites fibres, en particulier lorsque les examens en stimulo-détection sont normaux, en cas de topographie douloureuse atypique et/ou sans contexte étiologique évident. En cas de douleur neuropathique d’allure périphérique, l’analyse des seuils nociceptifs et des potentiels tardifs permet de confirmer l’existence d’un dysfonctionnement des fibres Aδ. L’exploration des fibres C en PEL reste délicate en routine mise à part lorsque l’on explore la face. Les PEL sont également indiqués pour mettre en évidence une dysfonction des voies spino-thalamiques en cas de lésion médullaire ou supra-médullaire. Ils sont particulièrement utiles lorsque l’imagerie est normale ou discordante avec la clinique, et lorsque les potentiels évoqués somesthésiques (PES) sont normaux. Les PEL ont également un intérêt médicolégal pour authentifier un dysfonctionnement neuronal lié à un contexte post-traumatique ou post-chirurgical qu’il s’agisse d’une douleur neuropathique centrale ou périphérique. Enfin, ils peuvent être utilisés pour faire la part entre des douleurs neuropathiques et des douleurs purement psychogènes.
Among non-invasive laboratory tests, laser-evoked potentials (LEPs) are the best tool for assessing dysfunction of small sensitive fibers and lesions of the spino-thalamic pathways. LEPs are useful to diagnose small-fiber neuropathy, especially when pain topography is atypical, in absence of obvious etiology, and when nerve conduction tests are normal. The analysis of nociceptive thresholds and the acquisition of late potentials enable routine exploration of Aδ fibers. The exploration of C fibers is more difficult in practice except for the face. LEPs are also required in case of central neuropathic pain without any spinal or brain MRI abnormalities and without SEP abnormalities. LEPs are sometimes the only test showing a dysfunction of the spino-thalamic pathway. LEPs can also discriminate pure psychogenic pain and patients with certain neuropathic pain.
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2011, Revue Neurologique
L’exploration des neuropathies sensitives à petite fibres (NSPF) nécessite l’utilisation de techniques spécifiques. À la différence de l’exploration quantifiée de la sensibilité thermique qui reste subjective, les potentiels évoqués laser (PEL) permettent d’étudier spécifiquement et de façon objective les fibres sensitives de petit calibre. L’exploration des fibres C par les PEL est possible mais pose encore de nombreux problèmes méthodologiques ; seule l’exploration des fibres A delta est effectuée en routine. En pratique, l’étude des seuils perceptifs et nociceptifs, l’analyse des latences et surtout des amplitudes des PEL permettent d’objectiver une atteinte des fibres A delta, de la quantifier et d’en déterminer l’extension (gradient distal/proximal). L’amplitude des PEL est corrélée à la perception de la douleur générée par le laser et au degré de déafférentation. L’intérêt et la sensibilité des PEN reste enfin à établir par rapport aux biopsies cutanées.
Standard neurophysiological techniques evaluate exclusively large myelinated fibers, but are not useful to explore sensory small fibers. Quantitative sensory tests have been developed to explore the thermal nociceptive function but this exploration is only subjective. Laser evoked potentials (LEPs) represent a noninvasive and objective test to explore thermal and nociceptive pathways. The clinical interest of LEPs have been assessed recently in the diagnosis of small fibers sensory neuropathies. In routine, the determination of detection and nociceptive thresholds, the analysis of N2P2 latencies and amplitudes enable demonstration of a dysfunction of A delta nerve fibers, to quantify these lesions and to determine whether the neuropathies are length-dependent or not. The LEP amplitude is negatively correlated to deafferentation. The interest of LEPs remained to be studied compared to skin biopsy.

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Original articleClinical application of laser evoked potentials using the Nd:YAG laserApplication clinique des potentiels évoqués laser Nd:YAG.

Abstract

Résumé

Introduction

Section snippets

Patients and methods

Results

Discussion

Clin Neurophysiol

Clin Neurophysiol

Clin Neurophysiol

Electroencephalogr Clin Neurophysiol

Neurophysiol Clin

Neurophysiol Clin

Pain

J Neurol Sci

Electroencephalogr Clin Neurophysiol

Electroencephalogr Clin Neurophysiol

Clin Neurophysiol

J Auton Nerv Syst

Pain

Neurophysiol Clin

Electroencephalogr Clin Neurophysiol

Brain Res

Pain

Clin Neurophysiol

Neurophysiol Clin

Clin Neurophysiol

Neurosci Lett

Pain

J Neurol Sci

Sensory involvement in spinal-bulbar muscular atrophy (Kennedy's disease)

Muscle Nerve

First pain event related potentials to argon laser stimuli: recording and quantification

J Neurol Neurosurg Psychiatry

Characteristics, detection, and modulation of laser-evoked vertex potentials

Acta Anaesthesiol Scand

Cortical responses characteristics to painful argon laser stimulation

Clin Evoked Potentials

Original article
Clinical application of laser evoked potentials using the Nd:YAG laserApplication clinique des potentiels évoqués laser Nd:YAG.